Radio frequency (wireless) communications systems are known in the art. Radio frequency communications systems typically include a transmitter and a receiver, such as for radio or television broadcasting. Other communications systems are bidirectional and include a first transponder that can send or receive a radio frequency communication, and a second transponder that can receive the radio frequency communications from the first transponder and transmit, via radio frequency, back to the first transponder. For example, cellular telephone communications are bidirectional.
Some communication systems include antenna diversity systems. For example, U.S. Pat. No. 5,203,024 to Yamao (incorporated herein by reference) discloses an antenna selection diversity receiver system for TDM signals that switches antennas for every assigned time slot so the best receive signal quality is obtained, according to a prediction made by using signal quality just before the assigned time slot.
U.S. Pat. No. 5,499,397 to Wadin et al. (incorporated by reference) discloses selecting one of at least two antennas in a communication unit. A signal is received by a radio frequency receiver during a receiving period from one of the antennas. The signal is transmitted in a digital communications format. The quality of the receive signal is determined by a switch diversity algorithm that bases the decision to switch antennas on predetermined thresholds established for each of: a received signal strength indicator, a phase error signal, and a recovered clock signal.
U.S. Pat. No. 4,584,709 to Kneisel et al. (incorporated by reference) discloses an antenna system that evaluates the quality of a received signal, and if the signal is below a predetermined threshold level, an alternate antenna will be selected. The antenna system will continue to sample the available antennas until an antenna produces a signal of acceptable quality.
U.S. Pat. No. 4,742,567 to Ohe, et al. (incorporated by reference) discloses an automobile antenna system including two antennas attached to the rear windshield of a vehicle body. A high-frequency pickup is provided on the front pillar of the vehicle body for detecting the lowering of the output level of a main antenna in advance, and an antenna selecting signal generator actuates a switch circuit so that the main antenna, the output level of which is predicted to dip after a predetermined lapse of time, is changed over to the sub antenna.
U.S. Pat. No. 5,097,484 to Akaiwa discloses transmission and reception equipment which is in digital communication with transmission and reception equipment having a single antenna An antenna for transmission is selected from a plurality of antennae at the time of transmission in accordance with information of a memory unit.
Radio frequency identification devices define another form of bidirectional communications systems. As large numbers of objects are moved in inventory, product manufacturing, and merchandising operations, there is a continuous challenge to accurately monitor the location and flow of objects. Additionally, there is a continuing goal to interrogate the location of objects in an inexpensive and streamlined manner. Furthermore, there is a need for tag devices suitably configured to mount to a variety of objects including goods, items, persons, or animals, or substantially any moving or stationary and animate or inanimate object. One way of tracking objects is with an electronic identification system.
One presently available electronic identification system utilizes a magnetic field modulation system to monitor tag devices. An interrogator creates a magnetic field that becomes detuned when the tag device is passed through the magnetic field. In some cases, the tag device may be provided with a unique identification code in order to distinguish between a number of different tags. Typically, the tag devices are entirely passive (have no power supply), which results in a small and portable package. However, this identification system is only capable of distinguishing a limited number of tag devices, over a relatively short range, limited by the size of a magnetic field used to supply power to the tags and to communicate with the tags.
Another electronic identification system utilizes an RF transponder device affixed to an object to be monitored, in which an interrogator transmits an interrogation signal to the device. The device receives the signal, then generates and transmits a responsive signal. The interrogation signal and the responsive signal are typically radio-frequency (RF) signals produced by an RF transmitter circuit. Since RF signals can be transmitted over greater distances than magnetic fields, RF-based transponder devices tend to be more suitable for applications requiring tracking of a tagged device that may not be in close proximity to an interrogator. For example, RF-based transponder devices tend to be more suitable for inventory control or tracking.